1. Field of the Invention
The present invention relates to the field of heat treating of parts, and in particular, to a transfer vessel to minimize unwanted oxidation of heat-treated parts during fluid bed heat treating and subsequent quenching.
2. Description of Related Art
Processes for improving the physical characteristics of metal parts (e.g., castings, forgings and the like) that require a controlled temperature experience of the parts and sometimes require controlled furnace atmospheres, are well-known and are referred to collectively as xe2x80x9cMetal Treatment Processes.xe2x80x9d Examples of these processes include carburizing, carbonitriding, case hardening, through hardening, carbon restoration, normalizing, stress relieving, annealing, among others.
Generally, these processes involve exposing a metal part to elevated temperatures in a furnace having a controlled atmosphere that either alters or maintains the chemical composition of the part. Following a heating experience in the furnace, the part is typically cooled in a quench medium to achieve the desired physical properties.
Fluidized bed furnaces are well known in the metal treatment arts for their advantages of rapid and uniform heat transfer, ease of use and safety. Examples of the use of fluidized bed furnaces for metal treatment processes are illustrated in U.S. Pat. Nos. 3,053,704 and 4,512,821. Metal treatment with a fluid bed furnace is often followed by fluid bed quench.
As known to those skilled in the art, a fluidized bed consists of a mass of finely divided particles contained in a chamber through which a gas is passed through a multiplicity of ports in the bottom of the chamber. If the velocity of the gas entering the bed is properly adjusted, the particles are separated and levitated and move about in a random manner such that the entire bed of levitated particles resembles a liquid phase in behavior. Such apparatuses are well known and their fundamental behavior has numerous applications. A typical bed is disclosed in U.S. Pat. Nos. 3,677,404 and 4,512,821 owned by the assignee of the present application and are incorporated herein by reference. In a typical configuration, the fluidizing gas enters a plenum chamber generally co-extensive with the bottom horizontal extent of the bed and directs the fluidizing gas through the ports. The gas rises through the bed during which the liquid-like behavior is imparted to the particulate medium.
However, a problem with a number of metal treatment processes is that when the metal parts are removed from the furnace environment at an elevated temperature, the surface of the parts must be protected from contact with another atmosphere, such as air, until the part is cooled below a maximum temperature, typically in a quench or cooling bath. For example, if the surface of the parts is degraded by oxidation when contacted with air at elevated temperature, it is necessary to protect the parts from this contact until the temperature of the parts can be reduced. This is especially problematic when transferring parts from the furnace to quench.
To accomplish transfer without the parts contacting oxygen, it is typically necessary to build a sealed enclosure over the top of the fluid bed furnace, the transfer mechanism, and the top of the fluidized bed quench vessel, to exclude the presence of oxygen. This enclosure is typically purged with oxygen-free gas to exclude air from the furnace and/or the quench vessel.
Another approach to minimize oxidation during transfer is to employ a mobile transfer vessel, which is first positioned and sealed above the loading aperture at the top of the furnace. The parts load is lifted vertically out of the fluid bed furnace into the mobile transfer vessel. The transfer vessel is equipped with a slide-gate door at the bottom, which is then closed. The transfer vessel is then moved to the quench or cooling fluid bed which is also fluidized with a gas phase that does not contain oxygen. The slide-gate door is then opened at the bottom of the transfer vessel and the parts load is lowered into the quench or cooling fluid bed. The parts are removed after being cooled to a temperature sufficiently low that they no longer require protection from an oxygen-containing atmosphere. These enclosures are frequently cumbersome from an operating point-of-view and significantly increase the capital cost of the heat-treating furnace and quench system.
Thus, there is a need in the art for simple and non-capital intensive method of protecting metal parts from oxidation during transfer from furnace to quench. Accordingly, it is an object of the present invention to provide such a method and apparatus for use in such a method.
The present invention provides a parts container for minimizing oxidation of heat-treated parts. The container includes: a heat-resistant vessel having an interior space and including oppositely positioned first and second apertures; a heat-resistant, porous support element fluidly connected to the first aperture thereby providing a bottom for the vessel; and disposed in the interior space a plurality of fluidizable granular solids and at least one heat-treatable part. The parts container can additionally include a conduit fluidly connected to the porous support element to facilitate movement of fluidizing gas into the interior space of the vessel. Likewise, the parts container can also include a second heat-resistant porous support element fluidly connected to the second aperture to provide a top for the vessel. Preferably, vessel of the parts container is a cylindrical body and is of metal. The first and second porous support elements are preferably heat-resistant screens. In another preferred embodiment, the interior space of the vessel includes a plurality of heat-treatable parts dispersed in the plurality of fluidizable granular solids. The heat-treatable part is preferably of metal. In another embodiment, the plurality of fluidizable granular solids are reactive with the heat-treatable part.
A method of minimizing oxidation during the transfer of heat-treated parts is also provided. The methods includes: providing a fluid bed furnace having a chamber for receipt of parts to be heat-treated; providing the above-described parts container; submerging the parts container into the chamber of the fluid bed furnace where fluidizing gas enters the parts container through the porous support element thus fluidizing the plurality of granular solids. Preferably, the method further includes the step of removing the parts container from the chamber thus defluidizing the plurality of granular solids in the interior space of the vessel, where the heat-treatable part becomes submerged in the defluidized granular solids. In a more preferred embodiment, the method further includes the step of transferring the parts container from the fluid bed furnace to a fluid bed quench, and includes the step of submerging the parts container in the fluid quench.
Advantageously, the apparatus and method of the present invention inhibit oxidation of heat-treated parts during transfer in an oxygen-containing environment without resort to sealed enclosures and sealed transfer vessels as presently used in the art. These and other advantages of the invention will become more readily apparent from the description set forth below.